User Exits for Devt. Class MEW - BAPIs
If you create a purchase order using the BAPI PurchaseOrder.CreateFromData (function module BAPI_PO_CREATE), you can use this enhancement to check all items and, if necessary, change them prior to saving.
All field checks then take place after this function exit has been
EXIT_SAPLMEWP_002: Customer Exit: Creation of Purchase Order via BAPI_PO_CREATE